Disable IK then re-enable with different pose
You may be able to achieve something similar to this with the auto ik setting, but I'm not sure as I don't really use it.

IK/FK switching while preserving pose is difficult in blender as it currently stands.

The only way I've seen it done is by having 3 sets of bones, one for the FK pose, one for the IK and one to actually deform the mesh. A driver is then created to determine which chain controls the deform chain. Scripts are then provided to snap the FK chain to the IK chain's position and vice versa.

The above technique is used in the Rigify rig that comes as an addon with blender.

Also as an aside, why do you want to enable IK after posing? This will cause the rig to use IK interpolation(straight line between poses) between poses rather than FK? Most of the time you will actually want FK interpolation(arcs between poses), the only time you normally want IK if the end of the chain is explicitly fixed (e.g. a foot planted on the ground), and in these cases it is often easier to pose by IK anyway.